1. go-smtp is replaced by a fork that reverts StartTLS removal.
2. SASL LOGIN is no longer supported by upstream go-sasl, readded disabled by default.
3. Updated endpoint code to match new go-smtp authentication interfaces.
4. certmagic repo had some renames
5. Minimum Go version increased to 1.23 to match dependencies.
Support for a subset of libdns providers is added.
Some are enabled by default (assuming they are popular ones). AWS and Google Cloud SDKs take up extra 10 MiB of executable size.
Only filesystem storage is supported as of now.
Closes#3.